The ability to adhere to low surface energy materials, e.g., materials with nonpolar surfaces such as polyethylene, polypropylene and other polyolefin films, is steadily increasing in importance due to the proliferation of such plastics in many components of manufactured products. While rubber-based pressure sensitive adhesive compositions (PSA compositions, or more simply PSA or PSAs) adhere well to low surface energy materials, these compositions often lack thermo-oxidative and ultra-violet (UV) light resistance. Acrylic-based PSA compositions, on the other hand, have excellent thermo-oxidative and UV light resistance and as such, their use has been widespread and growing. However, acrylic-based PSAs in general lack good adhesion to low surface energy substrates due to the relatively high degree of polarity in the acrylic backbone.
Conventional methods of improving the adhesion of acrylic-based PSAs to low surface energy materials include mixing the PSA with one or more tackifiers and/or plasticizers. Tackifiers are typically resins having a low molecular weight, e.g., between about 500 and about 3500 grams per mole (g/mol) weight average molecular weight (Mw), and a high glass transition temperature (Tg), e.g., above about 80 C, and they reinforce the PSA by lowering the modulus of the PSA. This, in turn, imparts a greater viscous flow to the PSA which, in turn, promotes higher bond strength between the PSA and its substrate.
The mixing of a PSA with a tackifier, however, is not without disadvantages. These include a reduction of high temperature, e.g., greater than about 150 C, cohesion, increased minimum temperature bonding, e.g., at least about 15 C, and a reduction of tack at low adhesive coating thicknesses, e.g., less than about 0.8 mil. The reduction of tack at low thickness is a result of the high Tg of the tackifiers which inhibits ready flow of the PSA at low thickness.
Plasticizers, while very efficient at promoting viscous flow, compromise the cohesive strength of the PSA, especially at elevated temperatures, e.g., above 60 C.
Other methods to promote PSA low surface energy adhesion include the incorporation of rubber-based macromers into the acrylic backbone of the PSA. See, for example, U.S. Pat. Nos. 5,625,005 and 5,578,683. While this method does improve the low surface energy adhesion of the PSA, the resulting acrylic-rubber based polymer still lacks good tack performance at low thicknesses. This means that tackifiers are still necessary and as noted above, the addition of a tackifier adversely impacts the high temperature cohesion of the PSA. Moreover, since the incorporation of the rubber must occur during the manufacture of the PSA, the flexibility to formulate a PSA composition at the point of application is lost.